Because of its high strength to weight ratio, fiber reinforced composite structures have become attractive for aerospace applications, such as parts for airframes and propulsion power plants. Molding of such parts has been relatively time consuming and labor intensive because of the need to position elements accurately in the mold and to process slowly to avoid porosity and other internal and surface defects during polymerization, cross linking or hardening/gelling of the matrix material.
During initial curing cycles in such known processes, there must be a careful balance between applied temperature and pressure: viscosity and flow of the matrix can affect the positioning and unwanted movement of the particularly placed reinforcing fibers or cloth. For example, too high a processing temperature will cause reduced gelation time and rapid increase in resin viscosity which together with fast closure of the mold will result in the fiber reinforced plies to be displaced from their preselected position and orientation. Conventional compression molding of laminated composite articles requires a strictly controlled temperature and pressure application to achieve void free moldings. The process has to be accurately controlled relative to the rheological behavior of the resin matrix. The mold has to be closed relatively rapidly initially to expel surplus resin during its low viscosity phase which allows the resin to flow without disturbing the fibers. As the resin viscosity increases the mold closure rate is slowed down aiming at closing the mold just prior to the resin gelation point. Closing too prematurely results in purging too much resin out of the laminated article resulting in porosity. Closing the mold too late results in incomplete consolidation which also creates porosity. All the critical processing parameters, associated with conventional compression molding, of mold temperature, mold closure rate, pressure application and resin rheological behavior have resulted in expensive computer controlled apparatus and methods. These attempt to achieve the sophisticated process parameters necessary to produce high quality, void-free composite articles. One such system is reported in U.S. Pat. No. 4,455,268-Hindricks et al, issued Jun. 19, 1984, the disclosure of which is hereby incorporated herein by reference. Some of the problems associated with the flow characteristics of resins as a function of temperature and viscosity are discussed in that incorporated patent.